1. Field of the Invention
The present invention relates to a high-pressure discharge lamp and to a method of fabricating the high-pressure discharge lamp.
2. Description of the Related Art
FIG. 1 shows a sectional view of a high-pressure discharge lamp of the prior art. High-pressure discharge lamp 101 shown in FIG. 1 includes bulb 102 made of quartz glass and two electrode assemblies 106 that are held at the two end portions of bulb 102. Each of electrode assemblies 106 is constructed such that electrode rod 103 made of tungsten, molybdenum foil 104, and lead-in rod 105 are connected together in a series by welding. The electrode rod 103 side of each of electrode assemblies 106 is inserted into bulb 102, and the electrode assemblies 106 are held hermetically in bulb 102 with the tip portion of electrode rod 103 extending into discharge space 102a of bulb 102. The portions at which bulb 102 holds electrode assembly 106 are referred to as sealing portions.
Methods of holding electrode assemblies 106 in bulb 102 include a pinch-sealing method and a shrink-sealing method. The pinch-sealing method involves heating and softening the portions that are to become the sealing portions of bulb 102 with electrode assemblies 106 inserted in bulb 102 and then pressing the softened portions to closely adhere bulb 102 to electrode assemblies 106. The shrink-sealing method involves evacuating the interior of bulb 102 with electrode assemblies 106 inserted into bulb 102, and then heating and softening the portions that are to become the sealing portions such that the softened portions are caused to shrink in the radial direction, causing bulb 102 to closely adhere to electrode assemblies 106.
Thus, the electrode assemblies are sealed directly to the glass bulb of a high-pressure discharge lamp of the prior art. However, depending on the sealing conditions, innumerable cracks can occur in the sealing portion of the bulb due to the difference in thermal expansion between the electrode assemblies and bulb when the bulb is heated in the process of sealing the electrode assemblies. In a typical high-pressure discharge lamp, several hundred atmospheres of pressure are produced in the discharge space when the lamp is lit up. The repetition of turning a high-pressure discharge lamp ON and OFF causes these cracks that occur in the sealing portion to progress, and this progression eventually results in the rupture of the bulb.
A high-pressure discharge lamp that is directed to eliminating this occurrence of cracking when sealing the electrode assemblies is disclosed in Japanese Patent Laid-Open No. H11-154491. In this high-pressure discharge lamp, a portion of the electrode rods of the electrode assemblies is sealed in advance to a glass part having the same composition as the bulb, and the electrode assemblies are then sealed to the bulb by way of this glass part.
As another example, Japanese Patent Laid-Open No. 2001-23570 discloses a high-pressure discharge lamp in which a peel layer is formed on the surface of the position of the bulb at which electrode rods are to be sealed. The electrode rods undergo greater contraction than the bulb during cooling in the step of sealing the electrode assemblies, but this peeling layer is provided for facilitating the separation of the electrode rods from the bulb at this time and prevents the formation of cracks in the bulb. Examples of the peeling layer in this publication include a metal thin-film, a metallic base, and an oxide film.
Nevertheless, the above-described high-pressure discharge lamps of the prior art have problems as described below.
First, although the lamp described in Japanese Patent Laid-Open No. H11-154491 can prevent the formation of cracks in the bulb when the electrode assemblies that have been sealed to the glass part are sealed to the bulb, it will be likely for cracks to form in the glass parts when sealing the electrode assemblies to the glass parts, resulting in the same problem as the previously described prior art.
In the lamp that is described in Japanese Patent Laid-Open No. 2001-23570, on the other hand, considerable difficulty is encountered in forming the peeling layers on the surface of the bulb at which the electrode assemblies are to be sealed. In addition, the formation of the peeling layer causes change in the internal capacity of the discharge space, and substances that are sealed inside the discharge space may enter the gap that occurs between the peeling layer and the electrode rods. As a result, the pressure in the discharge space may fall below the prescribed pressure and the prescribed luminance may be difficult to achieve. In addition, the material that is used to form the peeling layer may itself form an impurity and lead to a shortening of the service life of the lamp.
In addition to the above-described occurrence of cracks in a high-pressure discharge lamp in which the electrode assemblies are sealed directly to the bulb, the deformation of the molybdenum foil (metal foil) that forms a portion of the electrode assemblies may result in the problem of decentering of the electrode rods. Decentering of the electrode rods causes the arc discharge that occurs when the lamp is lit up to approach the inner walls of the bulb and therefore causes a local increase in the temperature of the bulb. This local increase in temperature leads to a loss of transparency of the inner wall of the bulb and a drop in the brightness of the lamp. In addition, the focal point of the lamp may shift, whereby the emitted light falls below the designed level and the prescribed brightness cannot be obtained.
As a construction for preventing deformation of the metal foil, a construction is disclosed in Japanese Patent Laid-Open No. 2001-23570 in which metal foil is sealed by a glass part. Although the metal foil is reinforced by the glass part in this construction, it will be yet likely for cracks to occur in the glass part when sealing the metal foil, as with the construction that is disclosed in Japanese Patent Laid-Open No. H11-154491.